Valve actuating mechanism with mechanical means for self-adjustment



June 15, 1965 s. F. amass 3,139,011

VALVE ACTUATING MECHANISM WITH MECHANICAL MEANS .FOR sELF-ADJUSTMENT Filed April 30. 1964 2 Sheets-Sheet 1 -50 ZEN-Z v June 15, 1965 s. F. BRIGGS 3,189,011

VALVE ACTUATING MECHANISM WITH MECHANICAL MEANS FOR SELF-ADJUSTMENT Filed April 30, 1964 2 Sheets-Sheet 2 Jg' INVENTOR.

MJWAM ATTORNEY United States Patent VALVE ACTUATING MECHANISM WITH ME- CHANICAL MEANS FOR SELF-ADJUSTMENT Stephen F. Briggs, R0. Box 1617, Naples, Fla. Filed Apr. 30, 1964, Ser. No. 363,778 17 Claims. (Cl. 12390) pressure and at a small fraction of the cost.

Noise in the valves of an engine is primarily the result of undue impact of the valve against its seat. With the operating train properly adjusted, the cam of the cam shaft will control closing as well as opening movement and will seat the valve securely but quietly.

One of the problems in any mechanical take-up is to provide'a device in which valve actuation is positive when under load and yet one which will operate effectively both to expand and to contract over-all length to compensate for expansion and contraction of engine cylinders and push rods. When the engine is cold, the requirements differ from those which are encountered when the engine is hot. If the train is relatively too short, the valve operation is noisy. If the train is relatively too long, it will hold the valve open with loss of power and burning of the valve and its seat. Adjustment takes place in the relatively load-free part of the cycle when I the valve is closed, the valve spring being under minimum tension.

The take up device comprises a cam and cam follower which respectively are mounted rigidly on, or are parts i of, the rocker arm and push rod. In the preferred construction, the cam is on the upper end of the push rod and the cam follower is a part of the rocker arm. The entire push rod is pivotally movable on the lifter and is subject to spring bias exerted on its upper free end in a direction to increase the effective length of the push rod. The advantage of this arrangement lies in the fact that it does not alter the effective radius of the rocker arm.

The arrangement has the further advantage that both the cam and follower are fixed components of the push rod and the rocker arm. Even the provision for resisting undesired lateral movement of the rocker arm may be incorporated in the design of the cam and cam follower- The spring or springs providing the bias for taking up'lash may operate radially outwardly from the post on which the rocker arm is pivoted or may operate toward that post, depending on the orientation of the cam.

I have disclosed alternatively usable structures in which the cam surface is on the rocker arm rather than the push rod. I have also disclosed various types of spring biasing arrangements.

There is an important and critical relationship between the valve spring pressure, the angle of the adjusting cam with reference to a plane normal to the axis of the push rod, and the tension of the biasing spring. Valve springs have values determined by the engine designer according to considerations such as the inertia of the parts of the valve operating train and the expected maximum r.p.m. of the engine. An increase in valve spring tension requires an increase in the tension of the cam biasing spring and may require a change in the angle of the adjustment cam. Another variable has to do with the relative length of the radii of the rocker arms at opposite sides of its fulcrum. Some engines are designed to increase valve movement in relation to push rod movement, thus requiring entirely different spring tensions and possible changes of cam angle. In view of these and other variables, it is not possible, on the basis of present information, to provide a specific formula. This, however, presents no serious problems, being a matter of engineering design, bearing in mind the fact that the action of the cam shaft on the follower and valve must be positive during valve opening and closing movement and yet self-adjusting when the train is relieved of load, so that the valve will not be held open but will be actuated to and from its seat quietly and without lash.

For example, it has been found that with a valve spring having 175 lbs. pressure in the closed position of the valve, and having a cam at an angle of 8 to the plane above referred to, best results are obtained with a cam biasing spring having 16 lbs. tension. The minimum bias of the cam spring which is acceptable was found to be 11 lbs. tension.

With no change in cam angle, but a decrease in pressure of the valve spring to lbs., the bias of the cam spring seemed most satisfactory when it was 9 lbs. and it ceased to be satisfactory at a minimum level of 7 /2 lbs.

Using the 90 lb. valve spring but increasing the angle of the cam to 10 from said plane, best results were achieved with a cam biasing spring having a tension of 12 lbs. A spring having a bias of 9 lbs. was found to be the minimum desirable for the purpose.

The factors which have been found to be most desirable include the following:

The operation should be positive in a valve opening direction without the intervention of a cushioning spring.

The biasing pressure exerted on the adjusting cam should be so related to the angle of the cam and the friction characteristics of the materials used that adjustment in a direction to lengthen the train will normally occur only when the system is relieved of load and will not continue to such an extent as to hold the valve open. It is believed that all required adjustments normally occur when the valve seats.

The adjustment should preferably involve bodily movement of the push rod rather than movement of any separate part.

Since, in any given installation, the adjustment cam angle is quite critical within a narrow range, it is desirable to maintain the cam surfaces rectilinear, or at least within the permissible range of angles, throughout the distance -of relative movement of the cam and cam follower during self-adjustment. The distance is quite substantial because the self-adjustment compensates for wear as well as for variation in expansion and contraction.

While a cam angle of eight to ten degrees from a plane normal to the push rod has been found satisfactoryin particular installations, I do not wish to imply that this angle is critical per se. It has been found satisfactory for use with particular springs. For other rocker arm and spring combinations, entirely different angles may be required. The point here made is that cam angle in relation to the tension of the various springs involved is undoubtedly critical and has to be maintained throughout the range of self-adjustment.

In the drawings:

FIG. 1 is a view in section fragmentarily illustrating a portion of an internal combustion engine and a motion transmitting train for the actuation of a valve thereof.

FIG. 2 is a detail view in the plane of FIG. 1 fragmentarily showing a modified cam follower and showing the push rod cam in a slightly different position with respect to the follower.

FIG. 3 is a detail view in transverse section on the line 3-3 of'FIG. 1. s

'FIG.4 is a detail view taken in generally horizontal section on the line 44 ofiFIG. 1.

FIG. 5 is a perspective view of the push rod cam shown in FIG. 2. s

FIG. 6] is a' view partially in side elevation and par tially ina section comparable to that ofFIG. 1 showing a modified push rod and rocker arm assembly embodying "the invention. l 7 n FIG. r7'is a detail view taken in section on the line 7-7ofFIG."6. s

' FIG. 8 is'a viewsimilarto FIG. 6 showing a further modified embodiment .of the invention.

. FIG. 9 isa detail view. taken in section on the line FIG. ,10is a fragmentary detail view showing a further modified cam and cam follower arrangement.

FIG. 11 is a view similar to'FIG. showing amissmodification. v 7 I I It-will be understood that although FIG. .1 shows an air cooledengine with an overhead valve, the invention is by nosmeans limited to engine usage. or to anyparticu-L in type of engine, As shown in 1,' theengine has a piston 12 reciprocable in a cylinder 14 which has radial and attached securely torthe 4, s V I upper end of the push rod 40. Bars 64 on thecam carrier 56 supportan anchorage pin 66 for a tension'spring 68 if a single spring is used, as inFIG. 2. If a single spring 68 is used, it may have its other end anchored to'the post 48 as shown in'FI G. 6.

Alternatively, a pair of tension springs 70 may have an intermediate loop 73 encircling the push rod asin FIGS. 1

Iand 4', the free ends of-springs 70 being connected to an- 7 chorage screws 72 on opposite sides' of the rocker arm as shown in FIGJI and FIG. 4; The springs 70 function as a single-spring and may actually be made of a single piece of spring wires:

Irr eitheracase the cam surface 60 reacts with the cam follower boss '58 of the rocker arm in such a manner that the tension of'the biasing spring 68 tends to increase the effective length of the pushrodto take up-play or lash in the motion transmittingsystem. As above indicated, it has been found desirable that the angle of the cam surface 60 with reference to a plane normal to theaxis of the push rod 40 shall be .within that narrow range in which," taking into consideration the tension of the various "springs, thepushrod*movement-controlled by the cam shaft will positively control both the opening and closing movement of the valve' without holding the valve open and with self-adjustment as herein described to take up lash when thetrain isun'der ruinimum load; In any I a given system',fthe angle-is critical within avery few de- .cooling. fins 16. The removable head'ls has an overhead valve 20 controllinga port 22 which'provides valve-controlled-communication between the cylinder 14 and the passage 24. It is immaterial to the invention whether the passage 24 beregarded as an exhaust or an inlet passage.

grees if the device is to function, as intended, in a manner suchthat the effective length of;,the push rod will be decreasedflas well as increased to meet. varying conditions such as those whichresultfrom temperature change.

The valve 20 is biased toward its;seat by thexusuab- 'valve spring 26-whichsis seated on thegcylinder' head and acts on awasher-28 carried by valve stem 30. The valve is periodically forced from its seat by the valve cam, 32 on cam shaft 34. The cam coacts in the usual manner The precise range depends on valve spring pressures and other design factors such as-rocker arm radii and the coefficient offriction of thecam and its follower.

FIG. 2 shows the push rod40 offset toward the'left from the position in which the parts "are illustrated in FIG. 1. This offset 'has occurred in opposition to the with a follower 36 guided in bearing 38. s The push rod 7 f 40 has its spherical extremity 42,'swiveled in a seatprovided by a'socket 44 in the end'of the follower 36.

Through means presently to bedescribed, the push rod '40 acts on the rocker-lever 46 to depress-stem of valve-20against the bias of the valve spring 26. l

The rocker arm 46 is pivoted intermediate its ends. Conventionally, it is either mounted to oscillate on a .pintle or is universally mounted on a post 48 which has a generally: spherical bearing head 50 engaged by a com- I plementary' concave bearing surface 52 formed centrally -of the'rocker'armr46. A nut 54 threaded-tothe upper end ofzp'ost '48 adjusts the downwardly convex bearing member 50. s

The selfiadjustirig mechanism which forms the subject of the present invention preferably comprises an arrangement whereby theupper end 'of the push rod is automatically'movable to and from the post 48 to vary its effective length by shifting the position of the cam surface 60 on cam carrier 56 (shown in detail in FIGS. 2,

bias of the cam spring 68 and it results in a reduction in the effective over-all length of the push rod, thereby compensating for a relative'increas'e in pushrod length such asgmight be attributable either, to push rod expansion or engine cylinder contraction. It is emphasized that. in referring to an increase in relative push rod length, it is not intended to indicate that there is necessarily an absolute increase; "In fact, the actual over-all length may be decreased despite movement of the cam 56 to the right as .viewedxin FIG; land FIG. 2. In

' other words, there may have been a contraction in the length of the push rod greater than the increase effected by camming action. The result of'the camming action is to adjust push rodlength to the requirements of the valve train in relation to the valve in the system as a whole. Thus, while every movement of the cam 56 inwardly or outwardly will' bring about a change in the over-all length of the pushrod, this change is to be 'measured not in absolute terms but only in terms of what the length of the rod'might have been in relation to 3 and 5) with respect to the cam follower'bearing surface 58. The follower maybe formed on the rocker arm {or on a prefabricated part welded or otherwise fastened integrallyto the rocker arm) and is seated on the cam; Oneadvantage of having 'the cam surface at the bottom of a channel .is tointerlock the cam and cam follower 'againstr'elative vdisplacement of the cam laterally of the rocker arm 46, while accommodating movementfof the guided, as by the sides of slot59 in guide arm 61 (FIG. 1), it is still advantageous tohavc the cam and cam follower interlocked in-this-manner.since, if the rocker arm has a swiveled mounting asshown hereiu,.it could cam 69 in a direction which is longitudinal with'respect s to the rocker arm. Even" if thepush-rod.is.otherwise f i which operates'in alchannel 78 of the rocker arm 460,

which channel has acam surface 600which performs the function that was performedby the cam surface 60 in the device shown in FIGS; 1-3 and 5. a The surface 600 isinclined toia plane to which the axisof the push s r0d;400 is normal. When the push ;rod is-acted upon become angularly displaced from registry with the cam if itwere not for this interlock. s s

The cam carrier 56'may conveniently be prefabricated by the'tension spring 680, which tends to draw the push rod toward the post 48, there will-be a tendency to takeup lash between the push rod'400 and the valve stem 30. However, the adjustment is achieved, in this instance, at the expense of a change in the radial distance between the fulcrum of the rocker arm and the point at which the thrust of the push rod is applied thereto. For this reason, it is preferred that the cam surface be on the free end of the push rod, as in FIG. 1, rather than on the rocker arm, as in FIG. 6.

As in the device previously described, there is an interengagement between the chanenl-shaped cam surface 600 and the cam follower 76 which precludes lateral displacement of the push rod with respect to the rocker arm in .a plane which, in the disclosed embodiments, is the plane in which the post 48 and the push rod 400 are disposed.

45 E. W. Cunningham 20380 Nite Pats. May 22 Thus far, all of the cam biasing springs disclosed have been tension springs. FIGS. 8 and 9 disclose an arrange- 'ment which is very similar to that of FIG. 6 except that the cam surface 601 is reversed in angle from that of FIG. 6 and the spring 631 is a compression spring acting outwardly away from the post 48. There is a tubular guide 80 riveted or otherwise fastened between the side flanges of the channel-shaped rocker arm 466 as shown in FIG. 9. The end 82 of the compression spring 681 may seat directly against the post 48. The other end 84 is guided on a stud 86 carried by a thrust member 88 channeled to seat securely against the side of the push rod 400.

FIG. shows an arrangement generally comparable to that of FIG. 6 except that the spring 682 is a leaf spring connected with the outer end 90 of the rocker arm 462 and having its resilient portion 92 engaged with the push rod 400 in a direction to thrust it inwardly in a direction I shown by the arrow 94.

'bias it in the direction indicated by the arrow 102.

In these constructions, as in the devices previously described, the push rod and rocker arm are interlocked against lateral movement but are freely movable longitudinally of the push rod to exert thereon a camming action of a character such that the bias exerted on the push rod is, in every instance, so related to the cam angle that the cam biasing spring tends to take up lash by increasing the relative length of the push rod. In every instance, also, the cam angle is sufficiently close to rectilinear so that the desired functioning occurs throughout the range of relative positions of the cam and its follower. In every instance, also, the adjustment results from bodily movement of the upper end of the push rod with respect to the complementary end of the rocker arm. The cam and cam follower surfaces are formed directly upon these parts. In the preferred constructions, this bodily movement does not result in any change of the effective radial length of the rocker arm between its fulcrum on post 48 and the point at which the pressure of the push rod is applied.

. In repeated tests, engines having valve springs of the type and strength normally used in commercial practice have operated perfectly when equipped with a valve actuating train having the self-adjustment features of the present invention. The invention is particularly useful in engines having aluminum cylinders where expansion and contraction are very great in relation to the lesser degree of expansion and contraction of a ferrous push rod. In the use of the device, the engine valve closes properly under all conditions and tappet noise is absent. In other works, a device made as herein disclosed functions to reduce the over-all length of the actuating train as well as to increase over-all length and operates automatically as required.

Some of the desirable considerations in connection with the invention are as follows:

It is desirable that for convenience of access and minimum manufacturing cost and for the purpose of minimizing changes in such standardized parts as valves, the adjustment should be made between the push rod and the rocker arm rather than at any other point. Another advantage of having the adjustment at this location is to avoid the heat to which the cam and cam follower would be subject if the adjustment were made between the valve stem and the rocker arm.

Secondly, it is desirable that the cam and cam follower be either formed integrally with or assembled integrally on the push rod and rocker arm, thus eliminating any loose parts. I

Thirdly, the cam angle should be fixed within narrow limits determined with reference to other constants of the valve operating gear. The cam angle should lie within the said limits not merely at one point but throughout the range of relative movement between the cam and cam follower, the cam surface desirably being substantially rectilinear within said range.

Fourth, although the broad concept of this invention contemplates that the cam may be formed either on the rocker arm or the push rod, it is preferred that the cam be formed on the push rod because, in that event, the cam follower is located at a fixed radius from the center of oscilllation of the rocker arm and hence the adjustment does not change this radius when the cam is functioning to effect a change in relative length of the push rod.

Fifth, it is desirable toprovide some means of interlocking the push rod and rocker arm against relative lateral displacement while accommodating movement of the free upper end of the push rod toward and from the center of oscillation of the rocker arm. Disclosures include the preferred use of interacting guide parts respectively carried by the push rod and rocker arm as exemplified by a channeled interlock between the cam at the free end of the push rod and the cam follower on the rocker arm for accommodating the required movement of adjustment and restraining the parts against relative lateral movement.

Sixth, the movable free upper end of the push rod is subject to the bias of cam operating spring means, said bias being directly related to the bias of the valve spring. As the valve spring pressure is increased, the bias of the camming spring is also increased, and vice versa. Various springs are disclosed for this purpose, the preferred arrangement using tension spring means connected between the push rod and the rocker arm itself.

It is always necessary to maintain a balance between the cam angle and follower, cam biasing spring and the valve operating spring. This balance is affected by the top rpm. of the engine and the inertia of the parts of the valve operating train. The hydraulic valve lifter is rather heavy and constitutes a large part of the inertia of the valve operating train. With my arrangement, a very light lifter may be used which very appreciably reduces the inertia of the valve operating parts. Consequently, a high rpm. engine can utilize a lighter valve spring. This improves the engine efliciency, as much less power would be required to operate the engine valves. It also reduces the wear on the gears operating the cam shaft.

I claim:

1. Mechanism for controlling the unseating and seating of a valve having a valve-seating spring, said mechanism comprising the combination with a rocker arm having mounting means on which it is oscillatable, and a push rod having an actuator and a free end for rocker arm oscillation, of means for the automatic self-adjustment of said mechanism both in a direction to increase the effective length of such mechanism and to decrease the effective length thereof, said last means comprising cam and cam follower means, one of which is fixed on the rocker arm and the other of which is fixed on the free end o-fthe pushrod; the cam having a surface which; through out the range of relative movementrequired for selfadjustment; lies within'narrow critical limits of angle-with respect to a plane towhich-the pushrod-is normal,- and,

means-biasing the. free end of the push rodin adirection transverse to its axis to 'eiiec'ti relative movementbetween the cam and camfollower in' a direction for increasing the effective length ofthe push rod, the carh inclination ment'of thevalveythe entire seating movement of the valve'subject tothe biasof said valveseating'spring being transmitted through t e cam and cam follower meansto the push rod-wherebythevalve seating movement is posi- I tively controlled by said actuator; self adjustment between the cam and carn follower means occurring when the, cam and cam follower means are substantially free ofload after the seating of the-valve. r f f I 2., Valve operating mechanism according to claim, 1 in which the cam means is on'therocker arm and the cam" follower means is on the push rod f- 3."Valveoperat1ng= mechanism according to claim 1311' which thecam means is on the pushrod and the {cam follower means is on the-rockerarm;

4. Valve operatingtmechanism according to claim 1 in which said camsurfac'e is substantially rectilinear throughout s aid'range of'relative, movement.

5'. 'Valve'operating mechamsrnaccording to claim 1-in -which the biasing meanscornprisesa tension spring and 'the inclination of said surface is such that movement of the free endof the push rod occursin a direction toward the-rocker arm mounting means to increase the elfective length of the push'rod. J i i j v p Y 6. Valve operating mechanism according to claim 1 in which the biasing means acts' away from the mounting means of the rocker arm.

7 7. Valve operating mechanism according to claim 1 further combination with means comprising hearing parts ii respectively mounted on the cam and'the cam follower means for guiding the push rod in the direction of such relative movement and for confining it against'movement 'laterally' of-said direction. i

8. Valve operating mechanism according to claim 7 in 'which the cam means has a channel, the bottom of which 7 provides-said surface and theisides' of which are parts of said guiding means. 1 4

9. Valve operating'mechanisrn according to claim '7, in which the guidingmeans. comprises a relatively fixed arm having portions lying at opposite sides of the push'rod and providing bearing'sur faces'between which the push rod is confined. 1 v I i r t 10. Valve operating mechanism according to claim 1 in which the biasingmeans comprises tension spring means having one anchorage connected with the mounting means,

a and anotheranc'horage connected with the pushrod.

11. Valve operating mechanism according .to claim 1 in which the biasing means comprises tension springs respectively connected with opposite sides of the rocker arm and having a .comrnon anchorage means connected with the push rod. V v i g j 12. A valve operating mechanism according to "claim "11in which the tension springsare connected with the "rocker arm at theopposite side of said mounting means "from the point of connection of said springs to the push rod.

13, Valve operating mechanism-for a valve having a amigo 11 T 8 1 1 fstem subject to -the bias 'ofa valve spring, said mechanism 3 comprising the combination -with -a stem-actuating rocker '-arm having mounting-means on-which it is oscillatztble, an'da push rod having an actuatoranda free end engaged .5 with the rocker arm for the oscillationlthereofv in a direction to actuate the stem in opposition to the bias of said spring, of means for the-automatic self-adjustment of said "mechanism and comprising a-cam fixedon the free end of the push rod having a'cam surface which, throughout 1Q its effective length, lies within a cniti'cal langle such that lash in lthe'mechanism is taken up land the overall length of said'mechanism-is reduced as required to prevent the valve from-being held open; camifollower means fixed on the rocker arm andengaged with said cam, spring means 5 connected'withthe free end ofithe pushrod for biasing the free end of thepush rod infa direction'transverse to its axis tending-to efiect relativemovem'entbetween the cam and-cam follower within the effective length of said surfacefor eliminating lash-in said mechanism, an anchorage to which the spring-means is. connectedgand means for guidingthe free end of thepush rod in the direction of ---such relative movement andfor confining it against mover'nent'laterally ofsiaid direction the angle of the cam surface throughout saidrangewbcing so related to the co- -Bfil6iei1t of friction betweenthe cam and cam follower 'means and 'to the bias'of thevalve-sea'ting spring'and to thebias of-the means-for biasingthe free end of the push rod, that throughout said range'thel'entire displacement of f't he' push rodby its actuator inavalve unseating'direction is transmitted positively to the rocker arm to effect predetermined unseating displacement of the valve, the entire i'seating movement. of/the valve subject to the bias of said valve seating spring being transmitted through the cam and cam follower means to the push rod ,wherebythe valve seating movement is positively controlled by said actuator; self adjustment'between the cam and cam follower means occurring whenthe cam and'cam follower means aresuhsta'ntially free of load/after the seating of the valve.

'14. A valve operating mechanism according to claim 13 in .whichtheimeans last mentioned constitutes parts respectively carried by the rockerarm and thepush rod.

15. 'Valve operatingmechanism:according to claim 13 -in which the anchorage is onsaidrocker arm'mounting means.

- .16. .Valve operatingrnechanism according to claim 13 a in which said'anchorage is on said rocker arm.

'17; Valve operating mechanism according to claim 13 in which'the bias of said spring'means is proportional to the bias to which the .valve 'stem is subject, and the angle :of said cam surface permits'the 'self-a'djustment in both 7. directions of movement of the free end of thepush rod, whereby the valve connected fwith' said stem will not rei'main open andclearance in said mechanism is eliminated.

I 1 References Cited by the Examiner UNITED STATES PATENTS 1,333,160 '3/20 Cline. 1,410,771 3/22, Strohl '123 9o 1,994,743 3/3'5 Anderson 123- 9o 2,019,792 11/35 ,Nieman 123 90 2,030,345 2/36. 'Zahodiakin 123-90 2 ,053,743v '9/3,6;Russell; 123 9.o 2,827,028. '3/58jDa'dd 123-9o 2,863,431 12/58 .Marietti 123 2,970,583 '2/61 Knoblock 123 9o 3,087,479 4/63 Thompson 123,-90

, FOREIGN PATENTS :9, 584,741 9/33 Germany.

FREDEJQENGEL'IHALER, Primary Examiner.

RICHARD B. WLLKINSON, Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,189,011 June 15, 1965 Stephen F. Briggs It is hereby certified that error appears in the above numbered patent reqiiring correction and that the said Letters Patent should read as correctedbelow.

Column 5, line 9, for "chanenl-shaped" read channelshaped line 15, strike out "45-E. W. Cunningham 20380 Nite Pats. May 22"; line 72, for "works" read words Signed and sealed this 30th day of November 1965.

(SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Atlcsting Officer Commissioner of Patents 

1. MECHANISM FOR CONTROLLING THE UNSEATING AND SEATING OF A VALVE HAVING A VALVE-SEATING SPRING, SAID MECHANISM COMPRISING THE COMBINATION WITH A ROCKER ARM HAVING MOUNTING MEANS ON WHICH ITIS OSCILLATABLE, AND A PUSH ROD HAVING AN ACTUATOR AND A FREE END FOR ROCKER ARM OSCILLATION, OF MEANS FOR THE AUTOMATIC SELF-ADJUSTMENT OF SAID MECHANISM BOTH IN A DIRECTION TO INCREASE THE EFFECTIVE LENGTH OF SUCH MECHANISM AND TO DECREASE THE EFFECTIVE LENGTH THEREOF, SAID LAST MEANS COMPRISING CAM AND CAM FOLLOWER MEANS, ONE OF WHICH IS FIXED ON THE ROCKER ARM AND THE OTHER OF WHICH IS FIXED ON THE FREE END OF THE PUSH ROD, THE CAM HAVING A SURFACE WHICH, THROUGHOUT THE RANGE OF RELATIVE MOVEMENT REQUIRED FOR SELFADJUSTMENT, LIES WITHIN NARROW CRITICAL LIMITS OF ANGLE, WITH RESPECT TO A PLANE TO WHICH THE PUSH ROD IS NORMAL, AND MEANS BIASING THE FREE END O THE PUSH ROD IN A DIRECTION TRANSVERSE TO ITS AXIS TO EFFECT RELATIVE MOVEMENT BETWEEN THE CAM AND CAM FOLLOWER IN A DIRECTION FOR INCREASING THE EFFECTIVE LENGTH OF THE PUSH ROD, THE CAM INCLINATION BEING SUCH THAT VALVE SPRING BIAS TENDS TO DECREASE SUCH LENGTH, THE ANGLE OF THE CAM SURFACE THROUGHOUT SAID RANGE BEING SO RELATED TO THE COEFFICIENT OF FRICTION BETWEEN THE CAM AND CAM FOLLOWER MEANS AND TO THE BIAS OF THE VALVESEATING SPRING AND TO THE BIAS OF THE MEANS FOR BIASING THE FREE END OF THE PUSH ROD THAT THROUOGHOUT SAID RANGE THE ENTIRE DISPLACEMTN OF THE PUSH ROD BY ITS ACTUATOR IN A VALVE UNSEATING DIRECTION IS TRANSMITTED POSITIVELY TO THE ROCKER ARM TO EFFECT PREDETERMINED UNSEATING DISPLACEMENT OF THE VALVE, THE ENTIRE SEATING MOVEMENT OF THE VALVE SUBJECT TO THE BIAS OF SAID VALVE SEATINGH SPRING BEING TRANSMITTED THROUGH THE CAM AND CAM FOLLOWER MEANS TO THE PUSH ROD WHEREBY THE VALVE SEATINGG MOVEMENT IS POSITIVELY CONTROLLED BY SAID ACTUATOR; SELF ADJUSTMENT BETWEEN THE CAM AND CAM FOLLOWER MEANS OCCURRING WHEN THE CAM AND CAM FOLLOWER MEANS ARE SUBSTANTIALLY FREE OF LOAD AFTER THE SENSING OF THE VALVE. 